Interfacial tension water-various liquids

A similar imbalance of attractive forces exists at the interface between two immiscible liquids. Table 6.1 lists surface tensions of various liquids and also interfacial tensions at the liquid/water interface. The value of the interfacial tension is generally between those of the surface tensions of the two liquids involved, except where there is interaction between them. Table 6.1 includes several such examples. The interfacial tension at the octanol/water interface is considerably lower than the surface tension of octanol owing to hydrogen bonding between these two liquids. 

In Table 5.4, values for the interfacial tensions of various water/liquid interfaces, as calculated by the models of Fowkes and Girifalco and Good, are compared with measured values. It is clear that Fowkes model gives good results for aliphatic hydrocarbons that interact with water by dispersion forces only. As soon as polar interactions (tt electrons, dipoles, hydrogen bonding, etc.) play a role as well, strongly... 

EXAMPLE 6.5 Estimation of Interfacial Tensions Using the Girifalco-Good-Fowkes Equation. The following are the interfacial tensions for the various two-phase surfaces formed by n-octane (O), water (W), and mercury (Hg) for n-octane-water, y = 50.8 mJ m 2 for n-octane-mercury, y = 375 mJ m 2 and for water-mercury, y = 426 mJ m 2. Assuming that only London forces operate between molecules of the hydrocarbon, use Equation (100) to estimate y d for water and mercury. Do the values thus obtained make sense Take y values from Table 6.1 for the interfaces with air of these liquids. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

In the previous section, we demonstrated the micrometer droplet size dependence of the ET rate across a microdroplet/water interface. Beside ET reactions, interfacial mass transfer (MT) processes are also expected to depend on the droplet size. MT of ions across a polarized liquid/liquid interface have been studied by various electrochemical techniques [9-15,87], However, the techniques are disadvantageous to obtain an inside look at MT across a microspherical liquid/liquid interface, since the shape of the spherical interface varies by the change in an interfacial tension during electrochemical measurements. Direct measurements of single droplets possessing a nonpolarized liquid/liquid interface are necessary to elucidate the interfacial MT processes. On the basis of the laser trapping-electrochemistry technique, we discuss MT processes of ferrocene derivatives (FeCp-X) across a micro-oil-droplet/water interface in detail and demonstrate a droplet size dependence of the MT rate. 

In processing petroleum emulsions, chemical treating compounds may be added to a crude-oil emulsion to produce desirable oil quality and remove water or inorganic solids. The most common types of treating compounds are referred to as emulsion breakers. Various mechanisms are postulated as to how emulsion breakers function, but it is clear that an emulsion breaker must reach the interface of an emulsified droplet and the surrounding liquid. At that point, an emulsion breaker disrupts the interfacial tensions between oil and water and allows the droplets to coalesce and settle by gravity. 

Certain compounds, because of their chemical structure, have a tendency to accumulate at the boundary between two phases. Such compounds are termed amphiphiles, surface-active agents, or surfactants. The adsorption at the various interfaces between solids, liquids and gases results in changes in the nature of the interface which are of considerable importance in pharmacy. For example, the lowering of the interfacial tension between oil and water phases facilitates emulsion formation the adsorption of surfactants on the insoluble particles enables these particles to be dispersed in the form of a suspension and the incorporation of insoluble compounds within micelles of the surfactant can lead to the production of clear solutions. 

A Summary of Variation of Interfacial Tension (IFT) for Various Organic Liquids vs. Water... 

Interfacial tension between water (a) and various liquids (p) at 20 °C... 

Table 18.2 shows values of the interfacial tensions between water and various liquids. Note that the interfacial tensions between water and those liquids that are close to being completely miscible in water (for example, n-butyl alcohol) have very low values. 

A perfect solvent to displace water should have the high density and low interfacial tension for water. It should also be nontoxic, inflanunable, immiscible with water, and chemically inert. As a displacement liquid, a variety of organic solvents can be used. Of these, freon F-113 is the most frequently selected because it is nontoxic (T.V.A.—1000 ppm), its density is 50% greater than the density of water, its boiling point is only 48°C, and the latent heat of evaporation at the boiling point is 146.5 kJ/kg. Unfortunately, freons are not environment-friendly, so their use is restricted in various countries. In either case, the system should be perfectly tight to avoid gas release. 

The contact angles of Millipore water were measured on both flat and patterned surfaces of the polymer substrates, containing or not photochromic SP molecules. When a liquid is in contact with a solid surface in static equilibrium with its vapor, the liquid may form a contact angle 9 with the surface (partial wetting) when the various interfacial tensions obey Young s equation ... 

Section II.D, each material was finished by several methods, such as lathing and polishing, in order to measure contact angles for various surface roughnesses. The test liquids were water and 15% ethanol solution for the measurement of the contact angle and water, four kinds of ethanol solution of different concentrations, and two kinds of machine oils for the measurement of the liquid-vapor interfacial tension. 

The interfacial tension can undergo significant changes if the polarity of the medium is altered, such as in the stability/coagulation transition caused by the addition of water to hydrophobic silica dispersions in propanol or ethanol [44,52,53]. Also, the addition of small additives of various surface-active substances can have a dramatic effect on the structure and properties of disperse systems and the conditions of transitions [14,16,17,26]. The formation and structure of stable micellar systems and various surfactant association colloids, such as microemulsion systems and liquid crystalline phases formed in various multicomponent water/hydrocarbon/surfactant/alcohol systems with varying compositions and temperatures, have been described in numerous publications [14-22,78,79,84-88]. These studies provide a detailed analysis of the phase equilibria under various conditions and cover all kinds of systems with all levels of disperse phase concentration. Special attention is devoted to the role of low and ultralow values of the surface energy at the interfaces. The author s first observations of areas of stable microheterogeneity in two-, three-, and four-component systems were documented in [66-68],... 

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